The present invention relates generally to a serial bus communication device, and more particularly to a serial bus communication device for use in a process control sample system.
In the process control industry, process analysis equipment is used to monitor and control chemical processes. Such process analysis equipment extracts samples of chemicals and analyzes the samples to evaluate a chemical process. When samples are extracted, the samples typically must be conditioned by a sample conditioning system before the sample can be properly analyzed. For example, a sample can be conditioned by altering the temperature, pressure, etc. of the sample to make the sample suitable for use by the analyzer. A sample conditioning system typically includes a large number of regulators, filters, gauges, flow meters and other mechanical components and is subject to high maintenance. Due to the high maintenance typically required for sample conditioning systems, it is desirable that electronic sensors be used so that maintenance can be predicted and reduced or avoided in the sample conditioning system.
The interior of a sample conditioning system is usually considered a hazardous area in which highly flammable gases are present. In order to implement electronic sensors in such areas and to connect the sensors to a process analyzer, special safety techniques must be used. One such technique is to place all sensors and electrical signals inside special explosion-proof housings which prevent the electronics from coming into contact with hazardous gases on the outside. This technique is common in other process electrical situations, but such housings are large, bulky, and expensive to install. Because the components in sample conditioning systems are relatively small, and are numerous, explosion proof housings become impractical, and therefore are not often used in process sampling systems. Another technique is to route electrical connections through an intrinsically safe (IS) barrier. An IS barrier is a device which is typically located at a border of a hazardous area and non-hazardous area to ensure that all electrical signals flowing between equipment in the non-hazardous area and equipment in the hazardous area are limited to an energy level below that which will ignite flammable gases. There are various IS standards set forth by various certifying agencies for a system to be considered IS. Such standards include International Electrical Commission (IEC) IEC 60079-11, Factory Mutual (FM) 3610, Underwriters Laboratories (UL) UL913, etc. However, this technique requires that each separate signal be individually connected through individual IS barriers. Although this is possible, the cost of installing IS barriers and connecting each electronic sensor separately through an IS barrier to a process analyzer is often prohibitive. Accordingly, an IS communication bus capable of connecting all of the electronic sensors to a process analyzer through a single IS barrier is desirable.
The need for a communication bus capable of connecting multiple electronic sensors through a single IS barrier is consistent with the New Sample/Sensor Initiative (NeSSI). NeSSI is an initiative which promotes the use of modular sample system component technology to implement sample systems that are associated with analytical process equipment. NeSSI Generation 2 involves intelligent control of modular sample systems by using electronic controls and sensors. In order to achieve intelligent control of modular sample systems, two-way communication must be established between the modular devices (e.g., electronic sensors) that make up the sample system and a control device. Accordingly, a digital communication bus is needed to connect the modular devices so that the modular devices can communicate using a particular protocol. Examples of protocols used to implement NeSSI sample systems may include Controller Area Network (CAN), I2C, RS-485 based protocols such as Profibus or FieldbusS, RS-232 protocols, etc.
Profibus and Fieldbus are conventional communication buses widely used in the process industries. These buses are designed to permit installation of sensors and other instruments on large equipment in plant situations. Typically, such instruments are mounted individually and are large distances (up to several hundred feet) apart. These communication buses are available in a variety of implementations, including an intrinsically safe implementation. However, the IS versions are only capable of serving a small number of devices per each single instance of the bus. To serve more devices, additional instances of the bus must be established which requires duplicated and costly bus control apparatus. Because these buses are intended for a few devices located large distances apart, they are not well suited to sampling system usage which involves a large number of devices located closely together. Furthermore, Profibus and Fieldbus provide communication between the devices, but require the devices to have intelligence to enable the communication and support an established communication protocol. Accordingly, these buses are not very effective for enabling communication between simple devices, such as simple electronic sensors, which do not have integrated intelligence. While these buses can in principle be used for sampling system interface, the cost and system burdens involved in using them makes them costly and impractical for the purpose.
In a sampling conditioning system, there may be many types of sensors or devices, made by different vendors and utilizing different communication protocols. Many of these devices are simple sensors that have little or no intelligence in the device itself. These devices may be located in a limited amount of space. Accordingly, an IS communication bus is needed that is capable of connecting a large number of power consuming devices with limited intelligence, in a physically non-cumbersome manner.